Mount Carmel, Pennsylvania, June 17, 1948, a United Airlines Douglas
DC-6 aircraft encountered an in-flight fire in the cargo hold while on
a flight from Chicago to New York. The Pilots responded to the fire warning
by discharging carbon dioxide extinguishers into the cargo hold followed
by the pilots lowering the nose for an emergency decent.

As a result of
the lowering the aircraft nose, the carbon dioxide, being heavier than
air accumulated in the cockpit and asphyxiated the crew. The aircraft crashed
into high voltage lines killing all 43 on board.